1. Field of the Invention
The present invention relates to a signal receiving system for mobile communications, and more particularly, to a combined beamforming-diversity system using adaptive array antennas for a wireless fading channel environment.
2. Description of the Related Art
Beamforming systems using adaptive array antennas are commonly used as mobile communications systems for military radars. Beamforming systems are emerging as popular third-generation portable communication systems for consumers. Beamforming systems provide a fixed array of antennas to track mobile transmitters and reduce cochannel interference (CCI). Code Division Multiple Access (CDMA) based systems may dominate the new generation of mobile cellphone systems. Such a multiple access technique may provide robust high-rate communication, enabling rich data services for mobile cellphone consumers. As increased bandwidth is demanded by those consumers, communications systems must provide even higher data rates within the same amount of radio spectrum, and therefore increasingly spectrally-efficient systems must be devised. One of the main strategies used to increase CDMA system capacity is the use of multiple transmit and/or receive antennas, known as beamforming antenna arrays. The throughput of code division multiple access (CDMA) systems, which is degraded by interference signals from other users, can be increased with the beamforming system. As the number of users increases, the performance and reliability of conventional beamforming CDMA installations may be degraded. A conventional beamforming system is disclosed in detail in U.S. Pat. No. 6,336,033.
FIG. 1 illustrates a wireless fading channel model of a conventional beamforming system. With respect to FIG. 1, when a receiving system of a base station BS with an adaptive array antenna receives communication information from a desired mobile station MS1, signal interference appears due to another mobile station MS2 or reflection sources C and D. Here, the mobile stations MS1 and MS2 include all types of wireless fading channel communication systems, such as mobile phones, wireless LAN cards, vehicle navigation systems, and the like. A general beamforming system is used in a wireless fading channel environment via an adaptive array antenna including tens of antennas arranged at constant intervals. Accordingly, the beamforming system has a better signal and interference-to-noise ratio (SINR) since many interference signals arriving at various angles, which originate from another mobile station MS1 or reflection sources C and D, can be eliminated.
The performance of such mobile communication systems is greatly influenced by a fading effect due to various reflection sources in a wireless channel and interference signals from other mobile stations (users). The beam shape (A) of a target signal received in the base station BS of FIG. 1 is affected by the beams B of interference signals generated by the fading effect. In the beamforming system of FIG. 1 using an adaptive array antenna, fading can be alleviated to some extent by eliminating interference signals reflected from relatively far away reflection sources C and D. However, the reduction of the fading effect using only the adaptive antenna array, as in this case, is quite limited. In an urban communication environment where a base station for most mobile communication systems is located high above mobile stations, most signals transmitted from the mobile stations are reflected near the mobile stations and are received by the base station at a very small direction of arrival (DOA). As a result, the beam A of a desired user's signal can be undistinguishable from the beams of the reflected signals having similar DOAs and undergoes phase interference with the reflected signals, so that the fading effect causing sharp signal fluctuations within a short time cannot be overcome. In addition, when the number of antennas in the conventional beamforming system is smaller than the number of mobile stations (users), the SINR is lowered due to a reduced degree of freedom.